One thing I don't think has been mentioned is there are often two fans.
Today's high-efficiency furnaces typically do not have induced-draft
flues, but have "forced draft" to get the combustion gases out of the
house.
The forced draft comes from the combustion itself in the case of the
"pulse" furnace. I believe Lenox brought the pulse hot-air furnace to
market and Hydropulse brought the pulse boiler to market.
If it's not a pulse furnace then the other forced-draft furnaces and
boilers use a fan to clear combustion gases. This means a hot-air
furnace has two fans - one for the flue and one to circulate the hot-air
throughout house. Boilers have a fan for the flue, and a pump to
circulate the hot water throughout the house.
For a fan-forced-draft furnace you ought to hear the following
sequence...
(1) The flue fan comes on for a purge cycle to clear the combustion
chamber of any explosive gases. This usually lasts for 30 seconds. If
it's a boiler, the water pump also comes on at this time.
(2) After 30 seconds the flame ignites and the bonnet/water begins to
heat.
(3) When the bonnet is hot enough, the circulating fan comes on.
(4) If the house thermostat becomes satisfied, the flame is shutoff as
well as the flue fan (and the pump, if boiler). The bonnet fan
typically continues to run until the bonnet temperature cools down to
the low-cutoff-temperature. Therefore you hear one cutoff for the
boiler (all off at same time) and two cutoffs separated by some time for
the hot-air (flue-fan off first, then bonnet fan off later).
(5) If it takes some time for the thermostat to be satisfied, the
circulating fan (or pump) continues to run, but the flame and flue-fan
cycle on and off as the bonnet (or water temperature) cycles back and
forth between the low limit and the high limit. Each time the low limit
is reached, the flue-fan starts first for a purge cycle, then the flame
comes on. Each time the high-limit is reached the flame and flue-fan
shut off, but the bonnet fan (or pump) keeps running.
The primary difference in cycling between hot air and hot water is that
the circulating blower usually stays running for a while beyond the
thermostat shut-off in order to cool down the bonnet whereas the water
pump generally shuts off with the thermostat.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu